Using the brush tool

Hold down pencil tool by left clicking and chose brush tool
Options menu appears across the top.
Chose thickness of brush and soft or hard - soft brush is darker in the middle
Hard brushes like a pencil

Mode normal, opacity 100 flow 100
Select a colour on bottom left

Decrease flow to 10% then if you draw over the top it gets darker


Selecting more brushes

Windows - brushes (F5)
Click on arrow on top right of small screen
eg assorted brushes - replace brushes with assorted brushes
You can create your own brushes too